Progressive image shift for a saddle-stitched document

ABSTRACT

Shifting a page image while it is still in compressed form. The page image is compressed into independent strips that are a full page in the fast scan direction, and a small number of scanlines in the slow scan direction. Then, to shift the page image, whole strips can be deleted in one margin and/or deleted from the other. If the number of scanlines per strip is small enough, the difference in a margin created by the change of one strip will not be apparent. This invention can be used to advantage when it is decided at print time that the output shall be in the form of a saddle-stitched booklet requiring two-up printing, folding and subsequent trimming of the outside edges to uniform alignment. Here, a progressive page shift can be introduced at the printer controller while the images are still compressed, instead of requiring the change to be done to the PostScript Master, which would require reRIPing or decompression, shift and recompression.

BACKGROUND OF THE INVENTION

A method of varying the size of a vertical margin while the page imageis still in compressed form by compressing the page data into narrowvertical bands, and either deleting or adding bands to vary the margin.

Compression and decompression are normally done line-by-line in the fastscan direction across an entire page. In the past, one problem with thisapproach was that if an error occurred somewhere in the page, the entirepage would have to be recompressed. To save time, the page would beseparated into independent bands that were compressed separately. At theend of each band, a check sum, or some other kind of error checkingcould be used, so that if an error occurred, only the band with theproblem would have to be redone. Other reasons to divide a page intocompressed bands was to use less memory, or to time multiplex a numberof separations to the different channels of a color printer.

In the printing industry it is sometimes required to print a document inthe form of a saddle-stitched booklet. To describe this in the form of anumerical example, assume that sheets of paper that are approximately 17by 11 inches are used to form a booklet that is folded in the middle andstitched at the fold to produce a booklet that is approximately 8½ by 11inches. Two or four 8½ by 11 inch page images are combined on the 17 by11 inch paper. In this case, the central sheets would protrude furtherthan the outside sheets, so the last step is to trim the outside edge ofthe booklet to make all pages even. But all page images should have anequal margin to the outside edge of the page, requiring that each pageimage, from the front of the booklet to the middle, for example, must bemoved further toward the center than the previous one on each 17 by 11sheet during printing.

Usually the decision is made to print a center-folded booklet early inthe printing process when the pages are still in PostScript form. It iseasy to adjust the margins prior to RIPing (converting PostScript imagedata to a raster). However, if the decision is made after RIPing, thenthe document must be RIPed again with the new parameters or it must bedecompressed from its final form, the margins adjusted, thenrecompressed. These operations are all time consuming. A saving in timecan be achieved if the shift can be accomplished without reRIPing andwithout a decompression and recompression cycle.

SUMMARY OF THE INVENTION

In this invention, the page image is compressed into narrow verticalbands, each of which contains a suitably small number of scan lines.Then, to shift an entire page horizontally, the proper number of bandsmay be added to one side and subtracted from the other while the data isstill in compressed form. Adding a band may require only the addition ofan encoded word signifying an all-white band at one margin, whiledeleting a band simply requires the deletion of a band at the othermargin. Thus, both are simply performed.

Of course, the bands must be in the vertical direction, and must besmall enough so that the change of one band does not make a visibledifference. For a numerical example, a band that is 8 lines wide, in a600 line per inch printer, would amount to 0.013 of an inch, below whatis visibly detectable by the naked eye.

An example of the coding that could be used for this invention would berun length encoding. An example would be a code that names the color ofthe first pixel in a scan line and the length in pixels, and thereafterdescribes the color and length of each run. Thus a scan line that startswith one white pixel, two black pixels and three white pixels would be“1W, 2B, 3W . . . ” Using this system, an added white band of 8 lines at600 pixels per inch and 8½ inches long to shift the image by 0.013 of aninch would be simply “5100W, 5100W, 5100W, 5100W, 5100W, 5100W, 5100W,5100W”.

The result is that the page image can be shifted right or left whilestill in the compressed form. The use of a band-offset table facilitatesadding or deleting bands. The band-offset table is a list of thestarting location of each band in the entire page image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the generalized relationships when viewing the imaged sideof the paper.

DETAILED DESCRIPTION OF THE INVENTION

The disclosed embodiment is an application of image shift hardware whichis implemented in an image handling module of the printer controller,and supports positive and negative shifts in the slow scan direction tothe image data while it is still in compressed form. This feature can beused for progressive image shift where each page in the job is shiftedby a different amount. This progressive shift, for example, can be usedfor making saddle-stitched booklets when printing two-up or four-up pageimages. In such a booklet the gutter (margin) for each page is differentfrom the previous, as is necessary for the pages to align at the outeredge of the booklet after trimming.

This system has the ability to combine page images in real time duringprinting to produce saddle-stitched booklets. ReRIPing is not required,even though the document was not RIPed for saddle-stitched printing, asis the usual case. This is because, in the normal case, the pages existin compressed form, and a margin change that requires the addition ordeletion of scanlines would require decompression before the change andcompression after. The point of this invention is to build the marginchange capability into the printer controller hardware. Then, in realtime, when the pages are being delivered to the printer, and while theyare still compressed, the margins can be altered.

This process is made possible by using a compression algorithm whichindependently compresses 8-line strips of the page. Then, at any latertime in the process while the data is still compressed, shift in theslow scan direction can be done by skipping strips or adding strips.

Adding a second page image for two-up printing in the slow scandirection is done by concatenating the strips of the second page imageafter the first page. The addition of gutter between page images is doneby inserting blank strips between the first and second page images. Theshifting is performed as part of the pixel output operation and takes noadditional time. Shift and gutter can be changed for each output page.

This process supports a shift function to allow page images of adocument that was previously RIPed and compressed for simplex orstandard duplex printing, to be printed and shifted in real-time withoutreRIPing and re-compressing the job. This process supports positive andnegative shifting in the slow scan direction in one-band resolution. Theshift function is set by programming registers in the printercontroller. The shift can be changed on a separation-by-separation basisfor different shift requirements between pages. For image data that isshifted off the page, this process expects software to trim off-the-pagestrips and not send them to the printer. Only the strip that straddlesthe leading or trailing edge of the page is sent.

Image shift in the fast scan direction can be included to adjust theplacement of the image on the paper in fast scan direction. The shiftfunction is performed by adding or deleting pixels from each scan line.This shift function is set by programming registers in the decompressionhardware in the printer controller.

The following describes the image convention and shift/clip operationsin positive and negative directions.

Image Convention

The image on paper is described with respect to the following spatialimaging conventions, which must be followed by all the PEs (PrintEngine). The image is a rectangular image space whose sides are parallelto the slow scan and fast scan directions of the PE. FIG. 1 shows thegeneralized relationships when viewing the imaged side of the paper. Thefirst pixel delivered to the image frame is located at the lower lefthand corner, as viewed from the start-of-scan edge; the fast scan andslow scan proceed upward and to the right, respectively.

Active image data is the image information that is intended to appear onthe paper. It is the responsibility of the printer controller hardwareto align the page image data within the image frame so that it is imagedon the paper.

The following parameters are needed to perform image shift operations:

1. Output pixels per scanline (OPPS): this is the total number of pixelssupplied to the PE in the fast scan direction. This number includes bothmargin pixels and active pixels. The resolution of OPPS is on a pixelbasis.

2. Output scanlines per page (OSPP): this is the total number ofscanlines supplied to the PE in the slow scan direction. This numberincludes both margin scanlines and active scanlines. The resolution ofOSPP is on a scanline basis.

3. Input pixels per scanline (IPPS): this is the number of pixelssupplied in the fast scan direction. An ASIC reads in pixels perscanline from decompressor chips according to this parameter. Theresolution of IPPS is on an 8-pixel basis due to the requirement ofJPEG.

4. Input scanlines per page (ISPP): this is the number of scanlinessupplied in the slow scan direction. The ASIC reads in scanlines perpage from decompressor chips according to this parameter. The resolutionof ISPP is on an 8-scanline basis due to the requirement of JPEG.

5. Active pixels per scanline (APPS): these are the actual non-whitevalid pixels marked on paper in the fast scan direction. Active pixelsare the remaining pixels in the scanline after performing shifting. Theresolution of APPS is on pixel basis.

6. Active scanlines per page (ASPP): these are the actual non-whitevalid scanlines marked on paper in the slow scan direction. Activescanlines are the remaining scanlines in the page after performingshifting. The resolution of ASPP is on scanline basis.

7. Start-of-Page margin (SOP): this is the white scanlines area at thebeginning of page. This parameter can be used to shift the image on thepage as used in shifting the image over the hole punch area. Theresolution of SOP is on a scanline basis.

The process will generate padding strips for the SOP region and at theend of page after active image area. The process begins sending activedata to the PE after the last padding strip in the SOP is sent. Theprocess completes sending active data to the PE after the last scanlinein the active area per page is sent.

The system expects to deliver active image data strips and trimcompletely off-page strips at the end of page. If the last stripstraddles over the trailing edge of page, the system will deliver activestrip data up to Active Scanline per Page count and discard anyremaining strips.

The system will generate padding strips at the end of page after theactive image area. For performance and real-time operation reasons, thesystem completely trims off-page strips at the beginning of page anddoes not send them out. If the first strip sent straddles over theleading edge of the page, the system will discard off-page scanlinesaccording to SOP count and send only the first strip. The systemcompletes sending active scanline data to the PE after the last scanlinein the active image area is sent and then fills in the rest of the pagewith margin strips. At the beginning of each scanline, the system sendsthe margin (white ink) up to Start of Scan count before sending activescanline data. And at the end of each scanline, the system discards theremaining pixels in the Input pixels scanline after the number of pixelsput out reaches the Output Pixels per Scanline count.

While the invention has been described with reference to a specificembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made without departingfrom the essential teachings of the invention.

1. A method of progressive image shifting for use in printing amulti-page document, comprising: for each document page: forming pageimage data into a plurality of independent strips, each of which is afull page in length in a fast scan direction and a number of scan linesin a slow scan direction; compressing each of the plurality ofindependent strips, the plurality of compressed independent stripsforming a compressed page image; determining an amount and a location toshift the page image on the page; shifting the compressed page image byinserting a compressed blank strip corresponding to the amount to shiftthe page image at the determined location on the page; adjustinglocations of the plurality of compressed independent strips according tothe location of the compressed blank strip, the compressed page imageand the compressed blank strip forming a shifted compressed page image;decompressing the shifted compressed page image, and printing thedocument; wherein printing the document comprises printing two documentpages per each sheet of output media, and further comprising: for eachpair of first and second page images: determining an amount and locationof gutter space to be provided between the first page image and thesecond page image; inserting a compressed blank strip corresponding tothe determined amount of gutter space at the determined location betweenthe first shifted compressed page image and second shifted compressedpage image forming a shifted compressed pair image; and decompressingthe shifted compressed pair image.